1. Field of the Invention
The present invention relates to a pneumatic tire having a block pattern, and in particular, a pneumatic tire comprising blocks having sipes and formed in various structures. Further, the present invention relates to a vulcanizing mold having a particular structure for manufacturing a pneumatic tire having a block pattern, and to a method of manufacturing a pneumatic tire using this mold.
2. Description of Related Art
A pneumatic tire which has sipes in blocks so that the running performance of the tire improves on ice- or snow-covered roads due to the edges of the sipes is known as a studless tire. The studless tire is used on vehicles which run on ice- or snow-covered roads. FIG. 44 is a cross-sectional view cut along a circumferential direction of a conventional block tire.
As shown in FIG. 44, a pair of sipes 104, which extend in the transverse direction of a pneumatic tire 100 (the direction orthogonal to the surface of the paper in FIG. 44), is provided in a block 102 in a central portion in the circumferential direction of the tire (the direction of arrow A in FIG. 44). The block 102 is divided by the pair of sipes 104 into a central area 106, which is defined between the sipes 104, and areas 108, which are located respectively at both sides of the central area 106. The width, in the circumferential direction of the tire, of the central area 106 is narrow compared to the width of the area 108. As a result, the narrow central area 106 (hereinafter referred to as "narrow area") is less rigid than the area 108 (hereinafter referred to as "wide area"). When the block 102 of the pneumatic tire 100 contacts a road surface and is subject to frictional force from the road surface in a direction tangential to the outer circumference of the tire, as shown in FIG. 45, the narrow area 106, having lower rigidity, is greatly deformed, and one of the sipes of the pair of sipes 104 closes while the other opens. As a result, the narrow area 106 is used as an edge, and the driving performance and braking performance improve as the coefficient of friction on ice increases.
As an example, Japanese Patent Application Laid-Open Nos. 63-279903 and 63-279904 disclose a pneumatic tire having blocks in which the ratio of the width of the narrow area to the width of wide area is less than or equal to 0.8.
However, with regard to the ground contacting pressure within the block, as shown in FIG. 45, because the deformation of the narrow area 106 is large compared with that of the wide areas 108, the ground contacting pressure of the narrow area 106 is small. As a result, the narrow area 106 wears more slowly than the wide areas 108. As the wear progresses, the narrow area 106 protrudes further than the wide areas 108 (see the imaginary or phantom line in FIG. 44). When the narrow area 106 protrudes further than the wide areas 108, the ground contacting pressure on the wide areas 108 decreases, and there is a drawback in that the on-ice performance of these wide areas 108 deteriorates. FIG. 46 is a graph illustrating the relation between the amount of protrusion of the narrow area and the coefficient of friction .mu. on ice. It is clear from the graph that as the amount of protrusion of the narrow area increases, the coefficient of friction .mu. on ice decreases. (When the protruding surfaces of the narrow area 106 and the wide areas 108 are even, the coefficient of friction .mu. on ice is set to an index number of 100). The concept of FIG. 45 was recognized by the inventor of the present application, and the FIG. 46 data was obtained from an experiment carried out by the inventor of the present application.
Further, because the groove width of the sipe 104 is narrow, when the sipe 104 opens, it is easy for stress to concentrate on the bottom portion 104A of the sipe 104. It is therefore easy for cracks to form from the bottom portion 104A. In order to combat this drawback, forming an enlarged portion, having a circular cross-sectional configuration, at the bottom portion of one sipe is known. However, as illustrated in FIG. 47A, when two sipes are disposed so as to be adjacent to each other, if an interval L between enlarged portions 105 is too narrow, a drawback arises in that the rubber which should be between the sipes 104 is caught and remains between blades 110 of a vulcanization mold 200, as shown in FIG. 47B. Further, the rigidity of the narrow area 106 between the sipes 104 decreases, and the amount of shearing deformation, when the pneumatic tire is subject to a front-to-back force F during running, increases. As a result, a drawback arises in that it is easy for cracks to form from the bottom portion 104A.
FIGS. 48A and 48B are plan views of conventional blocks having at least two sipes. When side force is applied to narrow areas 106A, 106B, 106C due to cornering or the like, distortion due to shearing occurs, and cracks form in the bottoms of the sipes 104. If the cracks worsen, depending on the case, the narrow areas may break off.
There exist a variety of configurations a plurality of sipes formed in a tire block. In the majority of cases, a pair of sipes is formed so as to be parallel. Examples are illustrated in FIGS. 49A and 49B.
However, when a side force during running, such as that described above, acts upon tires having a variety of configurations for blocks having sipes, the bottom portion of the narrow area 106 is deformed by shearing force in the transverse direction. Cracks are generated, and, depending on the case, the narrow area 106 may break off.
When the above-described blocks having sipes are disposed on a pneumatic tire tread such that the narrow areas are arranged in a row along the transverse direction of the tire, another drawback arises in that areas having low rigidity are arranged so as to coincide along the transverse direction of the tire. When a side, force is applied to the pneumatic tire, the deformation of the narrow areas is great, cracks form in the narrow areas, and the narrow areas break off.
Usually, concave portions for forming blocks are provided in a vulcanization mold for vulcanizing a block tire. After an unvulcanized green tire is placed in the vulcanization mold, the green tire is pressurized to a predetermined pressure and heated to a predetermined temperature. Due to this process, the rubber of the green tire is pressed tightly into the concave portions so that the outer contour is formed and vulcanization is effected.
FIG. 50 illustrates a block forming portion of a conventional vulcanization mold with the rubber of a green tire placed therein. When internal pressure is applied to the green tire and rubber 121 of the green tire is forced into the concave portion 100, the concave portion 100 is closed by the rubber 121, and air and gas generated by the rubber stagnate so that the rubber cannot flow. In order to prevent such a situation from occurring, vent holes 140, which communicate with the outside air, are provided in vicinities of the four corners of the concave portion 100 so that the air within the cavities can escape and the rubber 121 can flow more easily. When a pair of sipes is provided so that the block is divided into three areas, as illustrated in FIG. 51, it is necessary to form a pair of thin, plate-shaped blades 110, which extend from one wall surface 116 of the mold to the opposing wall surface 118, within the concave portion 100. The concave portion 100 is divided into three areas by the pair of blades 110 so that a small concave portion 112 is formed between the pair of blades 110. In this case, as shown in FIG. 52, when the rubber 121 of the green tire flows into the concave portion 100, there is no place within the small convex portion 112 for the air to escape so that the small concave portion 112 can be closed by the rubber. Therefore, the rubber does not flow to the bottom portion of the small concave portion 112, and there is a drawback in that bare areas exist in the block after vulcanization.
In order to eliminate this drawback, the inventor of the present application attempted a method of forming a pneumatic tire by using a vulcanization mold in which through-holes 114, shown by the imaginary lines in FIG. 51 (because the holes 114 are not statutory prior art), are provided so that the small concave portion 112 and end concave portions 115, which are adjacent to the small concave portion 112, communicate via the through-holes 114. However, this structure did not sufficiently allow the air inside the small concave portion 112 to escape, and consequently did not sufficiently prevent the formation of bare areas.